The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Generally, an automatic transmission achieving more speed stages has been developed to enhance fuel efficiency and drivability.
Such an automatic transmission with more speed stages is desired to improve power performance and driving efficiency while pursuing the engine-downsizing. Particularly, highly efficient multiple-speeds transmissions are desired to have excellent linearity of step ratios which is closely related to drivability such as acceleration before and after shift and rhythmical engine speed. These features are considered to secure competitiveness in automatic transmission field.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components increase, and as a result, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train which may have a small number of components may be important in order to increase a fuel efficiency.
A conventional eight-speed automatic transmission has gear ratio span of 6.5-7.5 (gear ratio span is an important factor for securing linearity of step ratios), and thus improvement of power performance and fuel economy may not be great.
In addition, if an eight-speed automatic transmission has gear ratio span greater than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may be deteriorated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.